The radiomagnetotelluric (RMT) method utilizes man-made signals generated by distant transmitters or by dedicated local transmitters. Man-made electromagnetic (EM) signals in the 1250-kHz frequency band come mainly from two sources: (1) distant radio transmitters operating in the 15250-kHz band and (2) nearby or distant industrial sources emitting either transients or higher harmonics of 50 Hz. The natural or background EM signals form a kind of noise floor. In Europe for the 15250-kHz band, there generally are sufficient transmitters available to estimate the EM transfer functions completely, i.e., the full impedance tensor and the tipper vector. We show examples of the variability of power spectra and azimuthal distributions of transmitters from sites in Sweden, Hungary, The Netherlands, and Spain. We also show that the estimated transfer functions are stable as a function of time; even under typical noisy conditions, simple stacking of spectra in narrow frequency bands provides good-quality estimates of transfer functions.

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